What about a small cabin/deckhouse? Perhaps it's a little bit extreme idea, but I have thought a little about a small cabin that could be positioned on board the TI. It could be big enough to house a not too big man, a couple of kids, a dog or a lot of equipment. It would be about the size of a fairly big car box. It could be strong and have a good lock. It might float in the water. It might have wheels. It could give a dry storage place. It might be good in case of various kinds of emergency. It might give you a shield from dangerous animals.

I have thought some more about this. There is a car box with the name Boatbox which can also be used as a rowing boat, made in England: http://www.boatbox.co.uk/. The main problem is that it weighs 25-30 kilograms and costs about 1200 dollars with delivery to Sweden. I can buy a new car box for about 200 dollars in Sweden that weighs only 10 kilograms, so I might go for that instead.

What are you hoping to achieve from adding such a large container? A waterproof capsule you can sleep in while at sea, or for camping? If the latter, I think you would be far better off with a hammock (search on here as there have been several successful ones done). The container you linked to would be too big to fit in the front seat area of a TI, being 1800mm long, so it could only go out on the akas.

Remember also that your hull has a very large volume, and you can buy drybags very cheaply on eBay. You get a double benefit is you load lots of stuff into drybags inside the hull, as this increases the flotation of the Island by reducin the volume of water which could fit in the hull.

Well, I explained that there could be several reasons to want a small cabin. One is that it can be locked. I want to have a small electric bike on the boat, which would be much more secure if the bike is locked into a roof box. I also have lots of other stuff which doesn't go into the hull, like for example the paddle.

If you fall on a bike you can easily damage both yourself and the bike. But not if it is in a roof box.

The Boatbox certainly seems big. There are smaller car boxes in many sizes, but the length of the car box is not much of a problem if it is situated a bit above the rear akas. You can still use the Hobie drive and all the other equipment, even if you have the car box above the front seat.